1. Field of the Invention
The invention relates to a process for reducing the wax content of wax-containing hydrocarbon feedstocks. More particularly, the invention relates to a process for converting wax-containing hydrocarbon feedstocks into high-grade middle distillate products including jet fuel having a low freeze point and/or diesel fuel and heating oil having a low pour point and a low cloud point.
2. Description of Related Art
Many liquid hydrocarbon feedstocks contain relatively high concentrations of straight chain and slightly branched chain aliphatic compounds having between 8 and 40 carbon atoms, commonly indicated as waxes. These compounds tend to crystallise on cooling of the hydrocarbon oil, a crystallisation which is quite frequently sufficient to hinder the flow of the liquid hydrocarbon and prevent it from being pumped or transmitted from one location to another. The temperature at which the hydrocarbon oil will not flow is commonly referred to as the "pour point." The temperature at which a cloud or haze of wax crystals is formed in the oil is commonly referred to as the "cloud point." These parameters are determined by way of standardised test procedures.
One way of converting such wax-containing feedstocks into high-grade products is by means of catalytic conversion, a process in which the waxes and other high-molecular weight hydrocarbon components are cracked in the presence of hydrogen to lower-molecular weight components. In this way middle distillates can be produced, and because of the ever increasing need for middle distillates, such as jet fuel, diesel fuel, and heating oil, it is of major importance to have good processes for their production, i.e., processes which effect efficient conversion of the objectionable high-molecular weight feedstock components to give middle distillate products with desirable properties.
Recently, such a process was put forward in U.S. Pat. No. 4,743,354. This document recommends a process for the preparation of middle distillates in which a waxy hydrocarbon feedstock of which a substantial proportion boils above 343.degree. C. is subjected to dewaxing or hydrodewaxing and the effluent is subsequently subjected to hydrocracking. The dewaxing catalysts employed are compositions containing a binder and a crystalline, intermediate pore size molecular sieve, the pores of which are defined by 10-membered rings of oxygen atoms, such as silicalite, zeolites of the ZSM-5 family, silicoaluminophosphates, and the like; the dewaxing catalyst may be provided with at least one hydrogenation metal. The hydrocracking catalysts employed are compositions containing a carrier, a Group VIB metal component and/or a Group VIII metal component, and an acidic cracking component, such as silica-alumina in combination or not with a large pore zeolite, e.g., X zeolite, Y zeolite, LZY-82, and LZ-10. Thus, it is stated, middle distillate products are obtained which have a low freeze point in the case of jet fuel and a low pour point as well as a low cloud point in the case of diesel fuel and heating oil. The present invention has for its object to provide a process which improves on the one known from said US patent specification. More particularly, the invention has for its object to provide a process by means of which, while using similar types of catalysts to those known from the aforementioned patent specification, middle distillate products can be made which have a lower freeze point in the case of jet fuel and a lower pour point as well as a lower cloud point in the case of diesel fuel and heating oil.
It has been found that these objectives may be attained by making use of a process in which the feedstock is subjected to hydrocracking and the effluent from the hydrocracking step is then subjected to dewaxing or hydrodewaxing: in other words, the sequence of steps followed is the exact reverse of that known from the aforesaid patent specification.